This invention relates to a monitor for an electronic TACAN beacon.
A TACAN beacon providing phase measuring bearing information for aircraft, as used since its inception, comprises a mechanically rotated antenna from which are transmitted two amplitude modulated signals of frequency 15Hz and 135Hz respectively. It is known to monitor the performance of a rotating TACAN beacon by means of one or more "far field" monitors, e.g. two diametrically opposed monitors. The field produced by a mechanically rotating antenna is not likely to be subject to directional distortion produced locally at the antenna due to the rigid mechanical devices used for pattern formation.
More recently there has been developed an electronic TACAN (ELTA) beacon in which the rotating field patterns are produced by electronic commutation of a static antenna array Typically such a beacon comprises a circular array of 16 antennae which are driven by e.g. Butler matrix feed circuitry.
The aims of the ELTA monitor are twofold.
Primarily the system is required to ensure that the Tacan pattern which is formed by the new ELTA antenna in the far-field is sufficiently correct so that, as far as is possible, bearing accuracy specification requirements are achieved by all receivers in all directions at all times.
Secondly, confidence in high quality far-field performance can only be achieved by an exact analysis of parameters at each antenna separately. Consequently the capability exists, and should be used, for initial setting up procedures. In particular basic mode phasing via logic controlled phase shifters is an essential pre-requisite for establishing coarse and fine pattern alignment in the far-field. Therefore the monitor should also be available as a powerful diagnostic tool.
The far-field pattern in ELTA is formed by the interaction of 4 separate RF components radiating simultaneously from the 16 antenna sub-arrays mounted on a cylindrical structure. The mutual phase relationship between individual antenna excitation and between the separate components themselves is critical. The 4 RF components radiated at any one time are a carrier component, 15 Hz upper and lower sideband components and a single 135 Hz sideband. As shown in FIG. 1 the field at any one point in space is strongly influenced by contributions from at least 10 antennas (11 in the diagram shown are within a range of 12 dB). It would therefore be impossible at that point in the far-field to analyse the received signals in such a way as to reliably identify partial or even complete faults in one of the involved antennas or its feed circuits.
Resulting from the fact that the complete Tacan pattern is formed by the synthesis of many components from a number of antennas the distortion of these components or failure of any of the antennas can generate direction dependent errors in the Tacan field. It is unfortunate that far-field monitors may detect no error in the direction in which they are situated yet quite substantial errors may exist in other directions.
It is felt, therefore, that satisfactory monitoring of ELTA patterns can only be effectively achieved by a close individual analysis of each antenna contribution. This might be considered to be a large, difficult and expensive task but the fundamental simplicity of the mode sequences which constitute the fundamental components of the ELTA pattern reduce the complexity of the problems a great deal. Furthermore the cyclic nature of the basic signal components permits very useful integration periods to be used thus eliminating the need for high precision sampling techniques. Signal-to-noise conditions are ideal and reference components for RF and low frequency are all available, derived from the basic Tacan system control. A valid prediction can be made of the far-field Tacan pattern if all the radiated components which synthesise that pattern can be individually and reliably analysed.